For the past century, environmental engineers have been using the activated sludge system and anaerobic digestion to successfully treat municipal wastewater (Metcalf & Eddy (2003). Wastewater Engineering: Treatment and Reuse. New York, N.Y., McGraw-Hill). In the United States, the nitrification process in the activated sludge system of public owned treatment works (POTWs) is very expensive with capital costs of the aeration basin alone valued at $26B and annual electricity costs of $335M (EPA (1996). Clean Watersheds Needs Survey (CWNS). W. D.C. Office of Water; Goldstein, R. and W. Smith (2002). Water & Sustainability (Volume 4): U.S. Electricity Consumption for Water Supply & Treatment—The Next Half Century. Palo Alto, EPRI; EPA (2007). Biological Removal Processes and Costs. W. D.C. Office of Water). More recently, the United States Environmental Protection Agency has proposed a stricter effluent nutrient limit (Total N of 0.82-1.73 mg/L and Total P of 0.069-0.415 mg/L) for Florida POTWs that is estimated to cost $24.4-50.7B in capital investment and increase annual operating expenses by $0.4-1.3B (Oskowis, J. (2009). Re: Numeric Nutrient Criteria Cost Implications for Florida POTW's G. C. Crist). Over half of the capital investment will upgrade the existing activated sludge system to an advanced biological nutrient removal (BNR) system. It is anticipated that the numerous extended aeration plants used for secondary treatment of wastewater in FL will be upgraded to Bardenpho 5-stage (BP5) systems or other advanced BNR system for biological nitrogen and phosphorus removal. The BP5 upgrade of the extended aeration plants will most likely not include the introduction of a primary clarifier and instead, the process will be operated with an elevated integrative resource recovery (IRR). The impact of the IRR on the microbial ecology of BNR systems has not been reported.
Researchers used empirical studies to identify key operating parameters for these systems to ensure effective performance. In order to increase the protection of surface waters from excessive oxygen demand from treated wastewater, engineers have modified the simple aeration basin to include anoxic and anaerobic zones and recycled nitrate-rich wastewater and anaerobic digester supernatant to promote biological nutrient removal. The anoxic zones allow for nitrogen removal by providing conditions with no dissolved oxygen and high nitrate levels for denitrification of nitrate rich wastewater. The anaerobic zones are necessary for phosphorous removal because anaerobic (no dissolved oxygen or nitrate) conditions are necessary for the phosphorous accumulating organisms (PAO) to release phosphorous and take up volatile fatty acids. These anaerobic zones are not used to generate methane.
The activated sludge system designs and operations are dependent on providing adequate biomass concentration in the aeration tank, environmental conditions for the biomass, and adequate time for the bioreaction. Carbonaceous biochemical oxygen demand (BOD) and ammonium are consumed as substrate by the heterotrophic and nitrifying bacteria, respectively. The conventional nitrification process requires a lengthy solids retention time (SRT), which prevents the washout of the slow-growing aerobic autotrophic microbes.
In addition to the aerobic, autotrophic bacteria (nitrifying bacteria), the growth of anaerobic, autotrophic bacteria (Anammox bacteria and others) and Archaea (methanogens) are also sensitive to the dissolved CO2 concentration. Like the aerobic, autotrophic bacteria, the Andrew's equation describes this sensitivity of their specific growth rate to the dissolved CO2 concentration. The anaerobic, autotrophic Archaea are thought to have a conserved metabolism with respect to the evolutionary time-scale, which may explain the similar Andrew's equation predictions for the optimal dissolved CO2 concentration for both bacteria and Archaea. In other words, the similarities in the predictions of the Andrew's equation for the optimal dissolved CO2 concentration for the specific growth rate of microbes may be the result of a common ancestor to both bacteria and archaea. This would predict that all autotrophic bacteria and archaea have similar autotrophic metabolism and dissolved CO2 sensitivities. This new knowledge can be useful for the improvement of the current practice for the treatment of wastewater (i.e., Anammox for nitrogen removal), sludges (i.e., methanogenesis for anaerobic digestion), and contaminated soils (i.e., dechlorinating bacteria and methanogens for complete mineralization of chlorinated organics) where anaerobic, autotrophic microbes are recognized as the rate-limiting step. In addition, landfill operation (i.e., methanogens) and biogenic methane production of coal beds (i.e., methanogens) may also benefit by optimization of dissolved CO2 concentration.
Anammox bacteria can be used in sludge treatment systems to treat anaerobic digester sludge supernatant that consists of high levels of ammonium. Typically, the SHARON reactor is used to oxidize one half of the ammonium to nitrite and the blend of ammonium and nitrite is fed to the Anammox reactor. Researchers have reported that the Anammox bacteria have a very long doubling time of 12 days. However, the operation of the Anammox reactor is at 5% CO2 in the headspace and controlled temperature (35° C.). This results in an elevated dissolved CO2 concentration, which inhibits the growth of these anaerobic, autotrophic bacteria. Operation at much lower and optimal dissolved CO2 concentration will reduce the doubling time to a few hours, which improve the performance of these bioreactors and may offer the opportunity for utilization of the Anammox bacteria for the treatment wastewater at ambient temperatures. This approach would reduce the capital and operating costs for BNR systems.
Current operation of anaerobic digesters for the bioconversion of organic solids to biogas (i.e., methane and carbon dioxide) exposes the biomass to the biogas. This exposure to the biogas controls the dissolved carbon dioxide based on Henry's constant, temperature, and the partial pressure of carbon dioxide (pCO2) in the biogas. The carbon dioxide concentration of the biogas for anaerobic digesters is typically between about 35 to about 50%. This range of gas phase carbon dioxide results in an elevated dissolved carbon dioxide concentration, which inhibits the growth of the methanogens. The methanogens are considered to be the rate-limiting step in anaerobic digesters with specific growth rate of 0.35 d−1 used for the design of the bioreactors. Anaerobic digestion of sludge typically requires the use of large holding tanks sized to accommodate enough sludge to account for a 20-day hydraulic retention time, which is required to maintain adequate biomass of slowly growing methanogens.
With proper control of the gas phase carbon dioxide level in the biogas exposed to the biomass, the specific growth rate of the methanogens can be increased substantially. For example, operation at 5% CO2 in the headspace for the direct production of biomethane (95-98% CH4; 2-5% CO2) increases the specific growth to about 2.36-2.92 d−1 compared to about 0.30-0.38 d−1 for conventional systems. The faster specific growth rate of the methanogens will allow for the design and operation of smaller anaerobic digesters for equivalent organic loading rates. With a safety factor of 5, conventional anaerobic digesters are operated at a minimum solids retention time (SRT) of 15 days, which corresponds to 0.31 d−1 for methanogens. With this invention and same safety factor, the minimum solids retention time for an anaerobic digester generating biomethane is about 1.71-2.12 days. This translates into a reactor that is between about 11-14% of the size of the conventional anaerobic digester used for the treatment of municipal sewage sludges. Operation at headspace CO2 concentrations lower than 5% will further increase the specific growth rate of the methanogens, but it may not be economically feasible due to the capital costs of the CO2 stripper and solids pretreatment technologies required for improved biodegradability of sewage solids. For operation at lower SRT, pretreatment of feed sludges or other organic solids that have low hydrolysis rates may be necessary. For example, the minimum hydrolysis rates for primary and waste activated sludge are 0.4 and 0.15 d−1, respectively. It is unclear whether the rapid rate of methanogenesis from the optimization of the soluble CO2 concentration may improve the rates of hydrolysis and subsequent fermentation by reducing the concentration of soluble and gaseous intermediates. A number of physical process technologies are available that treat the sewage sludges at high temperatures and/or pressures in order to generate soluble organics.
Alternatively, an existing anaerobic digester operating at an SRT of 15 days could be fed much higher organic loading rates than is recommended without inhibition of performance. This higher organic loading rate could be accomplished by either thickening the sewage sludges or adding other types of organic solids, such as food or paper wastes. These organic solids, especially the paper wastes, may require pretreatment to improve their biodegradability.
A biological approach improves the degradability of the sewage sludges prior to methanogenesis by the use of a thermophilic (55° C.) anaerobic reactor (acidogenic bioreactor) with a low SRT (2-3 days) for the express purpose of generating high levels of volatile fatty acids from the hydrolysis and fermentation of the sewage sludges. The Glendale wastewater treatment plant at Lakeland, Fla. utilizes this approach (2.1 days SRT in the thermophilic, acidogenic bioreactor) and generates low rates of biogas (37% CH4). Despite the low SRT, the thermophilic methanogens are still present and active in the system. The elevated CO2 concentration prevents the thermophilic methanogens from growing at faster rates. Operation at a lower headspace CO2 concentration will increase the specific growth rate of the thermophilic methanogens, which may eliminate the need for the downstream mesophilic anaerobic digester (methanogenic bioreactor).
Optimizing the specific growth rate of the Anammox bacteria and methanogens by controlling the soluble CO2 concentration also has great implications in the biological treatment of wastewater and sludges. For biological nitrogen removal, the control of soluble CO2 concentration is a new tool for designers of wastewater treatment systems that promise lower capital and operating costs. Rapid growth by Anammox bacteria by control of soluble CO2 concentration makes efficient nitrogen removal from wastewater at ambient conditions a possibility. The inventor has also discovered that the gas phase CO2 concentration can be optimized such that the rate of biogas formation is improved in the anaerobic digestion of sewage sludges. Increasing the specific growth rate of the methanogens by control of the soluble CO2 concentration by stripping CO2 from the collected biogas decreases the required solids retention time and in turn smaller digesters can be used which will save on capital costs. Heating the contents of the anaerobic digesters may not be necessary, if higher specific growth rates of methanogens are exhibited due to control of the soluble CO2 concentration, which will lower operating costs and make more biomethane available for other uses.